The invention relates to the field of low threading dislocation density relaxed mismatched epilayers, and in particular to such epilayers grown without high temperature.
The goal of combining different materials on a common substrate is desirable for a variety of integrated systems. Specifically, it has been a long-standing desire to combine different semiconductor and oxide materials on a common useful substrate such as a silicon substrate. However, just as the different materials properties are beneficial from the system application perspective, other properties may make such materials combinations problematic in processing. For example, semiconductor materials with different properties often have different lattice constants. Therefore, upon deposition of one semiconductor material on top of another substrate material can result in many defects in the semiconductor layer, rendering it useless for practical application.
Seminal work into the quantitative interpretation of dislocation densities and the connection to growth parameters was accomplished in the early 1990's (see, for example, E. A. Fitzgerald et al. J. Vac. Sci Tech. B 10,1807 (1992)). Using relaxed SiGe alloys on Si as the model system, it was realized that higher temperature growth of compositionally graded layers was a key to producing a relaxed SiGe layer on Si with high perfection.
However, practical constraints from current equipment used to manufacture Si-based epitaxial layers create difficulty in depositing high germanium concentration alloys without deleterious particle formation from either gas phase nucleation or equipment coating. For example, a large amount of current Si-based epitaxial wafers are created using single-wafer, rapid thermal chemical vapor deposition (CVD) systems. Although these systems offer great control of Si epilayer thickness uniformity across a wafer, the process encounters problems when germane is added to the gas stream to deposit SiGe alloy layers. Due to the lower decomposition temperature of germane than silane, dichlorosilane, or trichlorosilane, it is possible to nucleate germanium particles in the gas stream, or to coat parts of the reactor with thick deposits that can lead to particle generation.
The overall effect is that the epitaxial growth on the Si wafer can incorporate many particles, which not only degrade material quality locally, but also act as heterogeneous nucleation sites for additional threading dislocations, decreasing overall material quality. Particle generation occurs more rapidly with higher growth temperature; thus, the growth conditions that lead to lower threading dislocation densities (i.e. higher growth temperatures), unfortunately lead to more particles and a higher threading dislocation density than expected. Lower growth temperatures that avoid higher particle generation will create a higher threading dislocation density.